<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="stusys.css">
</head>
<body>
   我不在div里面
   <div id="topContent" style="background-color: aquamarine; height: 50px;" align="center">
       XXX系统
   </div>
   <div id="leftContent"  align="center">
       左侧菜单
       <table>
           <tr align="center"><td> <a href="stums.html" target="content">学员管理</a> </td></tr>
           <tr align="center"><td> <a href="teacherms.html" target="content">讲师管理</a> </td></tr>
           <tr align="center"><td>成绩管理</td></tr>
       </table>
   </div>
   <div id="rightContent" align="center">
      <iframe name="content" height="800px" width="90%">
          默认页
      </iframe>
   </div>
</body>
</html>